Client Requirement: Supported by many routers, check their site for complete list

Linksys firmware mod for the ever popular WRT54G and many many others, supports afraid.org natively.

IMPORTANT: This client works excellent! You must include your freedns.afraid.org update URL token in the "hostname" field for it to work. Please read DD-WRT with FreeDNS.afraid.org for setup notes. If you change your password, or delete/re-add your record, you will need to update your token within DD-WRT.

Client Requirement: Supported by many routers, check their site for complete list

OpenWrt is described as a Linux distribution for embedded devices.

It seems all one needs to do is a simple cut & paste from the Direct URL page after installing the luci-app-ddns package.

NOTE! The following instructions were submitted by member Robin Hansson July 08, 2013:
If you use luci, choose freedns.afraid.org as service and enter the corresponding values below. Here is the /etc/config/ddns file for those not using luci, or prefer to config this way:

I would like to let you know that I managed to get my Fritz!Box Fon WLAN 7270 working with freedns. Fritz!Boxes are very popular here within Central Europe

Product-Url: http://www.avm.de/de/

In the router settings, move to the Dynamic DNS tab, choose "user defined" for the dynamic DNS provider, paste the "Direct URL" from afraid.org into the "update url" field and enter your domain name, username and password and there you go!

This is a very friendly client I have tested. It was designed specifically for FreeDNS, so there is no complicated setup process, it automatically logs into the system and downloads your update URL list.

Important Notes:
Username must be LOWERCASE.
Password is CASE SENSITIVE.
Password is maximum length of 16 characters.

Support Windows XP, Windows 2003, Windows Vista, Windows 7, Windows 2008, SMTP Email Alerts for Critical Events when a record need to be updated or has failed. Advanced Event Logging System helps you track errors and report them back to us. Configurable Task Delay and Thread Priority (Multi-Threaded). XML Configuration File easy to understand. Very small program and service size, minimal memory usage.

This program is specifically designed to do dynamic dns updates for domains at FreeDNS (http://freedns.afraid.org). You only need to enter your username and password to start the updates. The update interval sets the amount of time between updates. You can also set the program to run at startup, and automatically minimize to the system tray.

This is a client for the afraid.org dynamic DNS service. If a cron job detects the external IP address has changed it connects to afraid.org and updates the DNS entries of all the domains of the given account.

This client is available in the YUM repo via : yum -y install afraid-dyndns

Update #3 2013-09-17 as noted by user Harald Brinkmann:
Using ddclient for afraid.org requires ddclient version 3.8.1 (that's the latest version available) and "protocol" *must* be set to "freedns". Using ddclient 3.8.0 produces weird results and a couple of protocol options sort-of work, but not really. I guess that that is the source of the problem Anki Borgh reported. Maybe you can update your info for the benefit of the next person struggling with setting ddclient.

Update #4 2014-07-23 Pásztor Szilárd reports that "username" must be lowercase or else you will receive "could not authenticate" error.